Nested KVM Issue

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Hello,

I have an OpenStack setup in virtual environment on CentOS 7.

The baremetal has *nested KVM* enabled and 1 compute node as a VM.

Inside the compute node I have multiple VMs running.

After about every 3 days the VMs get inaccessible and the compute node reports high CPU usage. The qemukvm process for each VM inside the compute node reports full CPU usage.

Please help me with some hints to debug this issue.

Thanks, Laurentiu

21 thoughts on - Nested KVM Issue

  • More details on the subject:

    I suppose it is a nested KVM issue because it raised after I enabled the nested KVM feature. Without it, anyway, the second level VMs are unusable in terms of performance.

    I am using CentOS 7 with:

    kernel: 3.10.0-327.22.2.el7.x86_64
    qemu-kvm:1.5.3-105.el7_2.4
    libvirt:1.2.17-13.el7_2.5

    on both the baremetal and the compute VM.

    The only workaround now is to shutdown the compute VM and start it back from baremetal with virsh start. A simple restart of the compute node doesn’t help. It looks like the qemu-kvm process corresponding to the compute VM is the problem.

    Laurentiu

    În dum., 14 aug. 2016 la 00:19, Laurentiu Soica a scris:

  • Hello,

    1.
    baremetalbrbm_1
    534e9b54-5e4c-4acb-adcf-793f841551a7
    104857600
    104857600
    36
    /machine

    hvm








    destroy
    restart
    restart

    /usr/libexec/qemu-kvm



































    2.
    [root@overcloud-novacompute-0 ~]# lsmod | grep kvm kvm_intel 162153 70
    kvm 525409 1 kvm_intel

    [root@overcloud-novacompute-0 ~]# cat /etc/nova/nova.conf | grep virt_type|grep -v ‘^#’
    virt_type=kvm

    [root@overcloud-novacompute-0 ~]# cat /etc/nova/nova.conf | grep cpu_mode|grep -v ‘^#’
    cpu_mode=host-passthrough

    Thanks, Laurentiu

    În dum., 14 aug. 2016 la 21:44, Boris Derzhavets
    a scris:

  • Reports posted look good for me. Config should provide the best available performance

    for cloud VM (L2) on Compute Node.

    1. Please, remind me what goes wrong from your standpoint ?

    2. Which CPU is installed on Compute Node && how much RAM ?

    Actually , my concern is :-

    Number_of_ Cloud_VMs versus Number_CPU_Cores ( not threads)

    Please, check `top` report in regards of swap area size.

    Thanks.

    Boris.

  • Hello Borins,

    1. So, in about three days after a reboot (this happened several times already) the compute node reports high CPU usage. It has 36 vCPUs and it reports a load higher than 40. Usually the load is about 2 or 3. The VMs qemu-kvm processes reports 100% CPU usage (for a VM with 4 CPU it reports almost 400%, for one with 1 CPU it reports almost 100%). The VMs are not accessible anymore through SSH.

    2. The baremetal has 2 CPUs, each with 10 cores and HT activated so it reports 40 CPUs. It has 128 GB RAM out of which 100 GB are for the compute node.

    I have 15 VMs running inside compute. They are summing up 40 vCPUs and 92
    GB RAM.

    There are no swap devices installed on the compute node so the reported SwapTotal is 0 KB.

    I’ll check is the memory on the compute gets exhausted as soon as the problem reproduces again (in about 2 days) but for now there are more than
    80 GB available.

    Note that a reboot of the compute node doesn’t fix the problem. Only a shutdown of the compute and a virsh start on it works.

    Thanks, Laurentiu

    În dum., 14 aug. 2016 la 23:27, Boris Derzhavets
    a scris:

  • I would attempt to decrease number of VCPUS allocated to cloud VMs.

    Say try 4 => 2 . My guess there is not enough VCPUs to run OS itself.

    I also guess CPU model < < Haswell. Please , confirm ( or not) if possible. Since Haswell was launched via my experience Intel Xeons based on this kernel (or latter kernels ) behaves much better then SandyBridge or IvyBridge based. Boris.

  • The CPUs are IvyBridge microarchitecture, Xeon E5-2670 v2.

    I could try lowering the vCPUs but I doubt it would help. Please note that the same VMs are running just fine (with a load of 2 out of an acceptable
    36 on the compute node) for about 3 days after a restart.

    În lun., 15 aug. 2016 la 08:42, Boris Derzhavets
    a scris:

  • Hello,

    The issue reproduced again and it doesn’t look like a swap problem. Some details:

    on the baremetal, from top:

    top – 08:08:52 up 5 days, 16:43, 3 users, load average: 36.19, 36.05,
    36.05
    Tasks: 493 total, 1 running, 492 sleeping, 0 stopped, 0 zombie

    %Cpu(s): 3.5 us, 87.9 sy, 0.0 ni, 8.6 id, 0.0 wa, 0.0 hi, 0.0 si,
    0.0 st KiB Mem : 12357451+total, 14296000 free, 65634428 used, 43644088 buff/cache KiB Swap: 4194300 total, 4073868 free, 120432 used. 56953888 avail Mem

    19158 qemu 20 0 0.098t 0.041t 10476 S 3650 35.6 13048:24 qemu-kvm

    The compute node has 36 CPUs and the usage is now 100%. There are more than
    50 GB of memory still available on the baremetal. The swap is barely used,
    120 MB.

    On compute node, from top:

    top – 05:11:58 up 1 day, 15:08, 2 users, load average: 40.46, 40.49, 40.74

    %Cpu(s): 99.1 us, 0.7 sy, 0.0 ni, 0.0 id, 0.0 wa, 0.0 hi, 0.1 si,
    0.1 st KiB Mem : 10296246+total, 78079936 free, 23671360 used, 1211160 buff/cache KiB Swap: 0 total, 0 free, 0 used. 78939968 avail Mem

    PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ COMMAND
    6032 qemu 20 0 10.601g 1.272g 12964 S 400.0 1.3 588:40.39 qemu-kvm
    5673 qemu 20 0 10.602g 1.006g 13020 S 399.7 1.0 1161:47 qemu-kvm
    5998 qemu 20 0 10.601g 1.192g 13028 S 367.9 1.2 1544:30 qemu-kvm
    5951 qemu 20 0 10.601g 1.246g 13020 S 348.3 1.3 1547:38 qemu-kvm
    5750 qemu 20 0 10.599g 990136 13060 S 339.1 1.0 1152:25 qemu-kvm
    5752 qemu 20 0 10.598g 1.426g 13040 S 313.9 1.5 663:13.65 qemu-kvm
    ….

    There are more than 70 GB of memory available on the compute node. All VMs are using 100% their CPUs and they are not accessible anymore.

    Laurentiu

    În dum., 14 aug. 2016 la 21:44, Boris Derzhavets
    a scris:

  • Running the compute node for several days simply triggers it.

    În mar., 16 aug. 2016 la 12:12, Boris Derzhavets
    a scris:

  • Enabled the logging on both compute and baremetal. Nothing strange in logs:

    on baremetal :
    Wed Aug 17 11:51:01 EEST 2016: committed 62310764 free 58501808
    Wed Aug 17 11:51:01 EEST 2016: 87025667 < 123574516 and free > 24714903, stop ksm

    on compute:
    Wed Aug 17 08:52:52 UTC 2016: committed 24547132 free 76730936
    Wed Aug 17 08:52:52 UTC 2016: 45139624 < 102962460 and free > 20592492, stop ksm

    and the compute node is again at 100% CPU utilization.

    În mar., 16 aug. 2016 la 15:26, Boris Derzhavets
    a scris:

  • Both baremetal and compute ? Are there any other metrics do you consider useful to collect for troubleshooting purposes ?
    În mie., 17 aug. 2016 la 13:04, Boris Derzhavets
    a scris:

  • For myself KSM is unpredictable feature. The problem is Compute, just this node

    does “copy on write” , so only Compute.

    My concern exactly is where would it lead to worse or better Guest behavior ?

    I am not expecting complete fix. I would track via top/htop and dmesg via Cron on 1-2 hr

    period.

  • I’ve tried with KSM disabled and nothing changed.

    I’ve upgraded KVM to qemu-kvm-ev. I’m waiting to see if there are any improvements and report back.

    În mie., 17 aug. 2016 la 15:10, Boris Derzhavets
    a scris:

  • No luck with qemu-kvm-ev, the behavior is the same. Running perf record -a
    -g on the baremetal shows that most of the CPU time is in _raw_spin_lock

    Children Self Command Shared Object Symbol

    – 93.62% 93.62% qemu-kvm
    [kernel.kallsyms] [k] _raw_spin_lock

    – _raw_spin_lock

    + 45.30% kvm_mmu_sync_roots

    + 28.49% kvm_mmu_load

    + 25.00% mmu_free_roots

    + 1.12% tdp_page_fault

    În joi, 18 aug. 2016 la 11:59, Laurentiu Soica a scris:

    Laurentiu Soica

  • I’ve tried the kernel-lt with same results. For kernel-ml, the baremetal wasn’t able to boot at that time.

    În mar., 23 aug. 2016 la 02:22, Adi Pircalabu a scris:

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